Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Glyphosate-resistant horseweed, Palmer amaranth, and common ragweed have all been identified in Delaware. In addition, many fields have plants of these species resistant to Group 2 herbicides; AND a few fields in the region have common ragweed resistant to glyphosate, Group 2 plus Group 14 (PPO-resistant).
Horseweed control in soybeans is very limited. Liberty (glufosinate) with Liberty Link soybeans; Synchrony with STS soybeans (but will not control ALS-resistant horseweed); for Roundup Ready soybeans FirstRate has been very inconsistent in our trials, but no other postemergence herbicide has sufficient activity on horseweed. Glyphosate will stunt horseweed and in situations with small horseweed plants or light infestations, this may be enough to provide acceptable horseweed suppression.
Palmer amaranth in corn can be controlled with a Group 27 herbicide plus atrazine. These herbicides include mesotrione (active ingredient in Callisto and Halex GT), Impact/Armezon, and Laudis. Liberty is an option with Liberty Link corn. Dicamba will control small Palmer amaranth in areas where it is appropriate to use. Control in soybeans is limited to Group 14 or Liberty with Liberty Link soybeans. Reflex (fomesafen) or Ultra Blazer should be used at 1.25 to 1.5 pt/A rate or Cobra at full rate. Other PPO-herbicides do not provide sufficient control. Flexstar GT contains the active ingredient of Reflex (fomesafen), but should be used at 4.5 pts to get the appropriate rate of fomesafen.
Postemergence control of common ragweed in corn can be achieved with atrazine, dicamba, or 2,4-D. The Group 27 herbicides (Impact, Laudis, Callisto) are not very effective for common ragweed unless they contain atrazine. In soybeans, metribuzin should be used at planting to provide some early-season control. Postemergence control for Liberty Link soybeans with Liberty is the only option if the field contains plants resistant to glyphosate/Group 14/Group 2 herbicides. If common ragweed is still sensitive to Group 14, Reflex (fomesafen), Ultra Blazer or Cobra can be used postemergence.
Liberty is a contact herbicide that needs good plant coverage to be most effective. Spray volume should be 20 gallons per acre or higher. Be aware that large spray droplets will reduce coverage. Application under high light intensity will improve control, so avoid applications in early morning or during evening hours.